Since the Crimson Hand is an extremely secretive conspiracy, I urge players to jump to the conclusion of this review in order to avoid the inevitable SPOILERS.

Still here? Okay! Essentially, the Crimson Hand is a conspiracy headed by a particular crafty individual Khaynite, who seeks to use his blood sorcery to once again make the resurgence of the First One empire a reality, but who may very well end up working WITH the PCs. How? Well, the mastermind Valos (CR 10 and 20 versions provided) has created a special spell that enables him to dominate/charm all individuals of a specific bloodline and now seeks to merge all kinds of different families in order to achieve supreme dominance - peace and coexistence may actually prove to be the undoing for the nations of Exodus, at least if Valos' complex plans come to fruition. Even worse, his political machinations have already shown promising results and, a s of yet, his elite cadre of deadly coconspirators have went under the radar of everyone. Supremely stealthy, hidden beyond layers upon layers of misdirection, the Crimson Hand makes for terrifying foes for smart players, especially if you're into intrigue. Not that the individuals don't stand their ground in battle:

Apart from Valos, the two other members of the inner circle are Lucia (CR 9 and 17), an Exodite killer of the highest caliber and Geist (CR 7 and 14) - and Geist rocks. The entity is an incorporeal, brilliant possessing, teleporting amoral construct that makes for a supreme spy and assassin indeed - most individuals will never know what hit them or that they even encountered the thing!

There also are 5 enforcers, i.e. assassins and executioners sent when special services are required - all are carefully created and stored and once one has been killed, the inner circle tends to reincarnate them. They all come with two incarnations as well and consist of two deadly crocodile-folk Kroca, an Exodite saboteur and an Aneishi-assassin/ranger. Of course, a trusty messenger is also one of the specialists included in the cadre of elite-enforcers of the crimson hand.

Two 3-level PrCs are provided for the elite Crimson Hand operatives - the casting Crimson Hand Acolytes (d6, good will and fort-saves, bad BAB, 2+Int skills per level, 2 levels spellcasting progression) can cast special sigils, which essentially are better symbol spells and an exsanguination ability, which not only kills grievously injured foes, but also heals the acolyte. The more martially-incline folks working for the Crimson Hand may take the Crimson Enforcer PrC (d10, 4+Int skills per level, good BAB, good Fort and Ref-saves), the ability to cast special spells as a spell-like ability and the option to gain AoOs against damaged foes and the power to vampirically heals themselves. They can also "Twist the Blade", which deals additional negative effects to his victims if he/she deals better than average damage on an attack. Along these two PrCs, we get 4 new feats and 7 new spells used by this elite cadre of dread conspirators. It is here that I should note something:
While the section with the PrCs and the feats and spells are tentatively headed as "player's options" the pdf mentions explicitly that the balance of these crunchy bits is intentionally on the upper hand. The Crimson hand is a small, elite cadre of individuals who take highest profile risks and don't have a lot of allies as well as the problem of having to stay secretive. If you do allow your PCs access to these classes, the additional risks have to offset the power of the content. Even though the pdf acknowledges this, the options are powerful with a capital "P":

Jugular Strike enables you to sacrifice sneak attack dice to potentially coup-de-grace foes - DC equals 10+ total number of dice of sneak attack + 1 per dice of sneak damage not used. OUCH! And there is a spell that is essentially an extremely powerful plot-device that makes an individual almost invulnerable. I'd usually harp on how overpowered these options are, but seeing that these individuals have managed to plague Exodus for as long as they have and their collective genius-level intellect, I can see them using these deadly abilities without feeling op or like cheating and best of all - the pdf acknowledges that some of these abilities should not end in player's hands. There are essentially plot-device spells and abilities herein which could severely unhinge a game when falling into player hands. The book does acknowledge the vast power these options provide, though, and seeing that the cabal has eluded the authorities for as long as it has, I can see the reasoning behind offering their operatives power beyond what would usually be considered appropriate.
While the PrCs are powerful, I'd deny my players access to the new (but very cool) spells and feats, though - they are clearly intended for these villains. Were I to compare the Crimson Hand to another sourcebook, it would probably be one of my favorite Scarred Lands-books featuring a certain shadowy secret society...

Conclusion:
Editing and formatting are not perfect, but can still be considered good - while I did notice some minor hickups due to auto-formatting like bold "Will"s in the text or superfluous tabs and the spells fail to specify what class they're available for (reading e.g. only "Level 3" instead of "Sorc/Wiz 3, Witch 4 etc...", the overall reading enjoyment was left intact for me. The pdf comes with extensive bookmarks and adheres to the beautiful 2-column layout of the NE-books.
As has been the tradition with LPJr-publications and specifically the NeoExodus-line, the artworks are simply stunning and each of the characters of the Crimson Hand actually gets his/her/its own mug-shot in gorgeous full-color.

The additional pieces of crunch herein that go beyond this highly political conspiracy have to be considered carefully and definitely are very powerful, but also utilize some rather innovative mechanics to do their work. As a result, the Crimson Hand feels like a shadowy cabal of supreme power, but one I'd discourage the DMs out there to let their players ally themselves with - at best, they should be pawns. Very expendable pawns who don't even know who they're working for...until it is too late. Writing-wise, only the Folding Circle surpasses this particular installment in the line in its cool ideas, which will also serve as my benchmark here - There are not as many statblocks provided for the operatives of the Crimson Hand as for the Folding Circle and a more extensive mythology of their exploits would have helped in creating scenarios involving this team of super-genius operatives. The crunchy bits beyond the statblocks are very strong and if you want to use the bits for your players, you might be disappointed.

If you're a DM looking for a truly deadly elite cadre of operatives/secret agents, you definitely should check them out, though. Thus, I'll settle for 2 verdicts - as a player-associated organization, this does not work well and should be considered 2.5 -3 stars, depending on the power-level of your campaign. The pdf knows this, though and if you're looking for a group to use against your players/smart antagonists with a deadly edge, you will probably really enjoy this one. For you, this is 4 stars or even 4.5. Seeing that I really enjoyed the Crimson Hand, I'll settle for a final verdict of 4.5 stars, rounded down to 4. Sometimes style and ideas are more important than rock-solid crunch, especially for NPCs.